In Situ Observation of Rapid Ligand Exchange in Colloidal Nanocrystal Suspensions Using Transfer NOE Nuclear Magnetic Resonance Spectroscopy

Abstract

Recently, solution NMR-based approaches have been developed that represent useful new tools for the in situ characterization of the capping ligand in colloidal nanocrystal dispersions. So far, this development has focused mainly on tightly bound ligands (no exchange) or ligands in slow exchange with the nanocrystal surface. In such systems, the ligand can be identified and its amount and interaction quantified via 1D (1)H NMR, (1)H-(13)C HSQC, and DOSY spectra. Here, we explore the case where capping ligands are in fast exchange with the nanocrystal surface. Using dodecylamine-stabilized CdTe (Q-CdTe|DDA) and octylamine-stabilized ZnO (Q-ZnO|OctA) nanoparticles, we first show that the NMR methods developed so far fail to evidence the bound ligand when the effect of the latter on the exchange-averaged parameters is marginalized by an excess of free ligand. Next, transfer NOE spectroscopy, a well-established technique in biomolecular NMR, is introduced to demonstrate and characterize the interaction of a ligand with the nanocrystal surface. Using Q-PbSe nanocrystals capped with oleic acids as a reference system, we show that bound and free ligands have strongly different NOE spectra wherein only bound ligands develop strong and negative NOEs. For the Q-CdTe|DDA system, transfer NOE spectra show a similar rapid appearance of strong, negative NOEs, thereby unambiguously demonstrating that DDA molecules spend time at the nanocrystal surface. In the case of Q-ZnO|OctA, where a more complex mixture is analyzed, transfer NOE spectroscopy allows distinguishing capping from noncapping molecules, thereby demonstrating the screening potential offered by this technique for colloidal quantum dot dispersions.

Nuclear Magnetic Resonance Spectroscopy Demonstrating Dynamic Stabilization of CdSe Quantum Dots by Alkylamines

“…This contrasts with the fast exchange rates determined in a NMR spectroscopy study on Q-CdTe stabilized by dodecylamine (DDA) and on Q-ZnO stabilized by octylamine (OctA). 16 In both cases, the QDs are observed to be dynamically stabilized by alkylamines, with an overall exchange rate exceeding 100 s -1 for Q-CdTe. In contrast to PL spectroscopy, NMR can directly address the ligands.…”

“…Importantly, ligands that only bind temporarily to the NP surface yield strong, negative NOE cross peaks similar to tightly bound ligands. 16 The 2D NOESY spectrum of pyridine-capped CdSe indeed shows these strong, negative NOE cross peaks for the pyridine resonances, which means that pyridine effectively acts as a ligand (see Supporting Information). However, diffusion-ordered spectroscopy (DOSY) on the pyridine-capped Q-CdSe yields a single diffusion coefficient of 2.11 Â 10 -9 m 2 /s for the pyridine resonances.…”

“…Apart from simply identifying and quantifying colloidal NP ligands, it also enables a direct assessment of their exchange behavior, which is classified as fast, intermediate, or slow relative to the relevant NMR time scale. Given the discrepancy in the exchange rates of alkylamines for Q-CdSe, based on PL, 12 and Q-CdTe, based on NMR, 16 we present in this Letter a NMR study of Q-CdSe stabilized by OctA that focuses on the ligand exchange regime.CdSe QDs were synthesized in a mixture of hexadecylamine (HDA), trioctylphosphine (TOP), and trioctylphosphine oxide (TOPO), with dimethylcadmium and TOP-Se as the Cd and Se precursors, respectively. 17 Two samples were studied, with average QD diameters of 4.4 (sample A) and 3.9 nm (sample B).…”

“…Apart from simply identifying and quantifying colloidal NP ligands, it also enables a direct assessment of their exchange behavior, which is classified as fast, intermediate, or slow relative to the relevant NMR time scale. Given the discrepancy in the exchange rates of alkylamines for Q-CdSe, based on PL, 12 and Q-CdTe, based on NMR, 16 we present in this Letter a NMR study of Q-CdSe stabilized by OctA that focuses on the ligand exchange regime.…”

Canopy Dynamics in Nanoscale Ionic Materials

“…Nuclear magnetic resonance spectroscopy (NMR) is frequently used to study molecular-level dynamics and diffusion in macromolecules and polymers, [27][28] and these NMR techniques can be extended to characterize composition, dynamics, and diffusion in nanomaterials. [29][30][31][32] We reasoned that these techniques could be employed to characterize dynamics in…”